


newton’s third law

by orphan_account



Category: Haikyuu!!
Genre: F/M, Pegging
Language: English
Status: Completed
Published: 2020-05-15
Updated: 2020-05-15
Packaged: 2021-03-02 23:13:50
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,975
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/24194917
Author URL: https://archiveofourown.org/users/orphan_account/pseuds/orphan_account
Summary: Your sex life, or whatever it’s called, with Keishin Ukai is excellent, you’ll be the first to admit. He’s the first man ever to make your voice hoarse from moaning. But the last thing you want is for him to get a big head over it. He’s annoying enough as it is, thanks.No, you need to get Keishin back down to Earth, somehow. He needs to be taught a lesson, taken down a peg.in which, keishin learns that every action has a reaction
Relationships: Ukai Keishin/Reader
Comments: 4
Kudos: 144





	newton’s third law

You frown at your reflection in the tiny rearview mirror, rubbing at the edge of a purple mark peeking out of your shirt collar. You hadn’t noticed it last night, but then again, you hadn’t really noticed much outside of Keishin calling you ‘Princess’ as he sat you in his lap and pressed a vibrator between your legs. And really, can you fault yourself for that?

Lucky for you (or rather for lucky for Keishin), you always carry a tube concealer in your purse, just for these types of situations. You pull out the tube and dab some concealer onto your tender neck, gently patting away the cream until it blends with the rest of your skin.

“Sorry 'bout that,” Keishin says from the passenger seat. You can see him from the corner of your eye, and he’s grinning like an idiot, which makes sense because he is an idiot.

“No, you aren’t,” you scoff, rolling your eyes. You need to get him out of your car before he makes you late for work, or worse, a student sees you with him. You pack the tube away, pulling out your lipgloss as Keishin shrugs unapologetically.

“Nope, not even a little bit. But really,” he says, leaning in closer until you can feel his breath on your ear, “can you blame me? Seeing you all marked up, having to hide my hickeys at school, it’s so hot.”

“Nice to know you’re turning into a caveman, Keishin,” you say. And blush because the heater is on and not because of how close he is, the bruise on your neck tingling, “but not everyone gets the luxury of working for our mommy. Some of us have real jobs.”

(Which, admittedly, is a low blow. Especially considering he coaches the boys’ volleyball team for practically nothing, and gives Karasuno students discounts on like half his inventory.) You purse your lips together to rub in the lipgloss, fighting back an apology.

“And yet, here you are,” Keishin notes, seemingly unruffled. “Hiding my artful love-bites under a layer of makeup. Real job and all.”

“Get lost, Keishin,” you say, rolling your eyes. You toss your lipgloss into your makeup bag and turn to him. “I have classes to teach.”

“Of course you do. Have a good day at work, Princess.” he says, and the ballsy bastard actually kisses you before getting out of your car. You give him your best-unimpressed glare, and his smile widens when he turns and sees your expression before heading into the store. 

And okay, yeah, maybe you a part of you is blushing and giggling on the inside like some idiot schoolgirl, but only because you’ve been treated like many things in your lifetime, from bitch to queen to child, but no one had ever made you feel like the Keishin does, like an actual, honest to God, princess.

But the other part is trying to figure out when he got so cocky, and how you’d allowed that to happen. Before you can contemplate further, a group of third-year students passes your car, and you put the car back into drive. Suddenly self-aware of how strange you must look mooning after the Sakanoshita Store guy, of all people.

You ponder it on the walk to your classroom, your sex life, or whatever it’s called, with Keishin Ukai is excellent, you’ll be the first to admit. He’s the first man ever to make your voice hoarse from moaning. But the last thing you want is for him to get a big head over it. He’s annoying enough as it is, thanks. 

No, you need to get Keishin back down to Earth, somehow. He needs to be taught a lesson, taken down a peg.

And just like that, it hits you. Throwing a glance at your class, who are all too busy with morning pleasantries to notice, you pull out your phone and do a quick google search, you find the article you’re looking for and skim it. You’ll need to do some after-school shopping, but you’ll gladly sacrifice that cute skirt from H&M for this. You put your phone away and neatly write a line of notes about the carbon cycle on the chalkboard, drawing a smug little smiley face in the corner. Oh, this is going to be fun.

Your next 'meeting’ (because what the fuck else was you supposed to call it?) with Keishin is on Friday, and today is Tuesday. If you stop at the sex shop tonight and get the supplies, you’ll have two nights to figure them out. Which is essential because the last thing you want is to be unskilled in front of Keishin. He’d never shut up about it.

The school day passes by in a blur. You faintly remember scolding Nishinoya for using Tanaka as a springboard and a brief conversation with Hinata about the ‘epic highs and lows of high school volleyball’. Also, the concept of mitochondrial DNA had been clunking around your headspace for most of the day which was odd because you don’t even teach biology. Still, mostly you were just focused on the tantalizing idea of giving Keishin a taste of his own medicine.

You drive to the sex shop two towns over, as opposed to the one just off the highway, partly because it’s cleaner, but mostly because there’s less of a risk of seeing someone you know. You’d hate to have a student catching you buying a strap-on. Oh, the rumors.

The salesperson is a heavily tattooed girl with electric blue hair and a black heart stamped on each freckled cheekbone. She’s really helpful, though. She takes her time explaining just how all the buckles work, and which dildo to buy to fit into which harness, so do your best not to judge her too harshly. She also recommends buying silicone-based lube over water-based lube, because apparently it lasts longer and isn’t harmful in anal sex the way it is in vaginal sex.

So you give her a five-dollar tip for her troubles, to which she responds by giving you the toothiest smile you’ve seen in your entire life and telling you your boyfriend has no idea how lucky he is.

Which you give her another three dollars for because she’s completely right. 

(About Keishin not knowing how lucky he is to have you. Not about him being your boyfriend, because he’s fucking not, okay?)

You bring your goodies home, feeling like you always feel after shopping: like you’ve just gotten a load of Christmas presents, and they’re waiting to be unwrapped. You have the presence of mind to hide the black and red bag in your oversized purse before entering your building. Just in case you happen to share the elevator with one of the old ladies on your floor.

Once you get into your apartment, you lock your door and layout your purchases on your dining room table, immediately picking up the dildo to test its weight. You’d picked a sparkly ribbed one, not because you particularly like it, but because you can’t wait to see Keishin’s face when he saw it. You’re pretty sure it’ll end up somewhere between shock, reproach, and begrudging amusement.

It’s the same abrasive yellow as Keishin’s bleached hair, average-sized, chosen more for entertainment value than anything else. You slot it into place then give the shaft an experimental tug to see just how well the metal ring in the harness holds it in place. Satisfied with the result, you examine the nubby, double-pronged vibrator on the opposite end of the harness. It’s supposed to go inside you when everything’s in place, so you get something out of it while you fuck Keishin senseless.

Though you’re reasonably sure that the very act itself of fucking Keishin senseless would have you curling your toes, you’re not about to deny yourself some extra stimulation.

You test the silicone lube between your fingertips. It feels weird, like the silicone-based face primer you’d used in high school, though this was less powdery and more expensive. You test on the skin above your knee, curious to see how long it takes to dry off.

While you wait, you take all of your clothes off, hanging up your blazer and throwing the rest in the hamper. You examine the harness, it’s an intimidating contraption of black nylon and silvery buckles, but that doesn’t deter you. You’re a high school science teacher, thank you very much. You explain chemistry to teenagers all day. This is nothing compared to that.

And actually, when you fit it onto your hips, it’s not too bad. A strap goes around each thigh, like a bikini, and one loops around your waist. You tighten the straps and peer down at the yellow, glittery penis now hanging heavily at the apex of your thighs. Huh. So this is what penises are like?

You grip the base and stroke up, grimacing at the sensation of your hand skidding over the rubber. Oh. Lube. Right. You squeeze some lube onto the dildo and start stroking again, much smoother this time. You hate how good the angle is; no wonder guys get so picky about handjobs. You fist it for a few minutes, feeling the vibrator bump against your clit. Which, considering its not even on, has no right to feel that good.

Once you get used to the way the dildo moves within its ring and how to compensate for the way the straps shift on your hips, you take the strap-on off and clean the dildo of lube. The stuff is way better than water-based lube, and you can’t wait to see it in action. You pack the strap-on and the lube back into the bag and leave it in your bedroom. Then you take a seat at your dining room table, pulling out a stack of ungraded papers instead. Time to spend some quality time with Marie Curie.

The next two days are validating, if nothing else. Keishin’s decided to go full little shit and keeps sexting you in the middle of your lectures like you’re supposed to just be able to explain oxygen theory of combustion after receiving a text detailing just how hard his cock is. You’d given him your best glare and sent a lengthy email telling him to fuck off, but to no avail. Plus, yesterday, he showed up at your office hours after practice, covered in sweat, and looking ridiculously hot, “just to say hi.” You won’t let it bother you, though. He’ll get what he deserves soon enough.

By Friday afternoon, you’re a mass of nerves and vindictive anticipation. Keishin’s been shooting you heated smirks all day. At lunch, he purposefully spills a packet soy sauce all over his hand just to seductively lick it off each of his fingers. You think it really speaks to your libido that, under the righteous indignation, you were actually pretty turned on by that. Stupid fucking Keishin, getting you hot and bothered with convenience store dumplings, of all things.

You’re practically vibrating when you open the door to your apartment at seven sharp, tamping down on your anxiety. You give Keishin your most relaxed, most expectant smile, and he responds by giving you that stupid(ly sexy) smirk and thrusting a bottle of cheap wine your way.

“Hey, Princess,” he says, bending down to peck you on the cheek. “How was your week?”

“Um,” you blink at him owlishly, thrown, “fine?”

“Really?” Keishin asks, stepping into your apartment and closing the door behind himself. As soon as the lock clicks into place, he’s on you like a starfish, head tucked into your neck. “Because mine’s been torture. All I can think about is how gorgeous you look under me. Over me. Everywhere. God, you drive me nuts.”

You feel something heavy in your chest. You bring your hands up to card through his hair. “I know the feeling.” Because all jokes and exasperation aside, Keishin’s under your skin in a big way, pumping you full of something that tastes like burnt, thick sugar and smells like Valentine’s Day chocolates. You’re drowning in Keishin Ukai, and you fucking love it.

“Do you now?” Keishin stills, then his hands change directions on your back, one scooping down to you ass and the other up into your hair. “And how does it feel, Princess?”

Oh, and there’s the smarmy little imp that’s been harassing you in school. Your lips curl into a devilish smile, out of Keishin’s line of sight, and you lean your weight into his hold. “Oh, I’m not sure I can even explain it, Keishin,” you sigh woefully. “Maybe I should just show you instead.”

“I think I could get behind that,” he agrees, pulling back. “Maybe even literally.” He leers down at you, eyes dancing with mirth.

“Classy, Ukai.” You snort despite yourself. “Remind me why I ever agreed to have sex with you?”

“Is that a request or an invitation?” His hands fall to your hips, thumbs rubbing lazy circles into your hipbones, “I accept both.”

You purse your lips, whether to fight a grin or a scowl, you’re unsure. “Let’s take this to the bedroom,” you suggest. “I have a surprise for you.”

“A surprise?” Keishin grins. “Lead the way.”

You set the wine bottle on the table and lead him by the hand to your room, hips swaying, nerves were forgotten. This is going to be so much fun. You open the door to your room, watching Keishin leap onto the bed. “Close your eyes and take off your clothes,” you order, unbuttoning your blouse. Keishin inhales sharply, eyes falling shut as he peels off his shirts and wiggles free from his pants. He’s already half-hard, boxers just beginning to tent.

“Can I open my eyes yet?”

“Not yet, no,” you replied, opening the drawer and pulling out your bag of tricks. you slid the strap-on into place, tightening the buckles with confident, practiced accuracy. “I thought we’d try something different today. Just the thought of it has kept me wet all week.”

Keishin twitches in his boxers, fists clenching on the edge of the bed. “Now, I’ve got to know. ”

“Open your eyes.”

Keishin blinks them open, freezing when they land on the dildo. You stroke it slowly, delighting in the way a ruddy blush works up his toned chest.

“Oh,” he says, sounding faintly disappointed. “I thought….”

“You thought you could tease me all week at school and get away with it,” you supply, baring your teeth when he flinches. “Newsflash asshole, every action has an equal and opposite reaction. So, what do you think of my cock, Keishin? I picked it out special, just for you.”

Keishin shudders, bowing his head in supplication. “Tell me what to do,” he says, voice gone hoarse.

“Answer the question.”

“It’s, uh,” Keishin stammers, glancing up at it, “it’s very… pretty?”

“Damn straight, it is,” you growl, striding toward the bed in long, slow steps. “What are you going to do with such a pretty cock, Keishin?” And wow, where is this coming from? You’re just supposed to fuck him and get it over with. This aggression is all-new, but you’d be lying if you said it didn’t feel good. And, judging by how hard Keishin is, you assume the feeling is mutual.

“Can I suck it?” he asks meekly, eyes pointedly not meeting yours. A total display of submission. You approve. You move to stand in front of him, positioning the cock at his lips, quirking an eyebrow at him. 

Keishin groans, reaching out to suck the head into his mouth. He bobs his head, working deeper down your shaft each time. You bite your lip, feeling a hot wave of arousal work down your spine. He’s beautiful like this, cheeks hollowed around the length of yellow, sparkly rubber. Your hand leaves the base to cup the back of his head, and his hand takes its place. He pulls back to suckle at the head, eyes looking up at you heatedly.

Fuck.

“So pretty,” you sigh, hand petting the dark hair on the nape of his neck. “I can see why guys like this so much.” Keishin’s eyes flutter shut, lashes long against his cheekbones. “What do you think, Keishin? Do you like sucking cock?”

Keishin moans, sucking as deep as he can go. When his eyes meet yours again, they’re desperate. His free hand moves to his own cock, pulling it out of the gape of his underwear.

You freeze, pulling his head back by the grip in his hair. “Did I say you could touch yourself?” Keishin shoots you a pleading look, but you’re already pulling out of his mouth, dildo shiny with spit. “Take them off, get on the bed. Hands and knees.”

He stumbles to do your bidding, cock dark red and angry-looking. You pick up the lube from where you’d placed it on the nightstand and kneel behind him. The lube opens with a wet click that makes Keishin jerk in surprise. You spread the lube liberally on your fingers, reaching out to trace one over his hole, teasing. Keishin mewls and pushes back, eagerly. You feel another gush of heat between your legs, pushing the finger in slowly. You work the finger in and out, curling it down to find his prostate. You find it on the fourth try, judging by the way he keens and clenches around you.

The second finger is met with a little resistance, and Keishin takes in a deep breath to relax his muscles. You kiss the small of his back in praise, scissoring the fingers once you’re able. This is a lot more intimate than you’d expected it to be, working Keishin open like this. It fills you with a strange sense of responsibility, you want to do this right, you want to make it good for him.

“Just relax, Keishin,” you whisper, as he whines and clenches around your third finger, “you can do this. We can stop anytime you want.”

Keishin heaves a great, shivering breath, but he relaxes. You work as slowly as you can, pushing against his rim more than thrusting in until he’s loose enough to take you. You squirt more lube onto your fingers, pushing them slowly into him until he takes them all the way to the knuckle. You make sure to graze his prostate every few thrusts, only content when he’s moving back to meet you thrust-for-thrust.

“M'ready,” he whispers, sounding wrecked. You pressed a kiss his hipbone in sympathy. “Want you.”

“Okay,” you say softly, pulling your lube-slick fingers out of him. You lube up your cock quickly, pressing the tip to his rim. “You sure?”

“Do it, Princess,” he says, wriggling his hips, “or I’ll start bringing bananas for lunch.”

You huff out a laugh, rolling your eyes. “Idiot.” You hold the cock firmly in one hand, pressing it carefully into him. His breath hitches and stops, and he leans into the intrusion. You press a wet kiss to the back of his neck when the head slides in. “How’s that?” You ask, moving slowly until the base of the dildo is pressed against his ass.

“Gimme a minute,” he manages, shoulders locked with tension. You hold your position, rubbing soothingly over his back and down his flanks. After a minute, he moves, shoulders relaxing. “Go slow, okay?”

You murmur an “okay” and pull out an inch. You move back in, starting a rhythm of tiny thrusts. You only lengthen them when he grows impatient and flails a hand at you. You pull out almost all the way, then shove back in, gasping when the vibrator buzzes to life over your clit.

You begin moving in earnest, grinding into him to feel the vibrator flutter against your clit. God, it felt good. You shift to the right a little, and Keishin moans, all high and whimpery and divine. You move to hit that spot again, grinning when he chokes out another moan. You angle yourself so that all of your thrusts will meet that spot, draping yourself over his back to work a hand on his cock. He’s hard as a rock and dripping pre-cum as he twitches under your touch.

Keishin makes a broken sound and works his hips, thrusting back onto your fake cock and forward into your fist. You feel the world spin around you; this was by far the hottest thing you ever done with anyone.

And you think Keishin might agree because thirty seconds later he starts babbling:“ fuck, I’m gonna cum. Shit, feel so perfect inside me, please, let me cum, tell me I can cum, please. I need you to say yes, please.”

You suck in a breath through your teeth. He wants you to give him permission? Oh, fuck, yes. “Cum for me, Keishin, wanna see you cum around my cock,” you command, voice deeper than you’d ever heard it. Keishin whimpers, and he’s cumming, hips spasming. You watch his hole clench around your cock and feel yet another gush of heat, this one dripping down your thighs. You continue to move inside him until he gasps and pulls away. You pull out slowly, groaning at the way his skin tugs around the length of you.

He flips onto his back as soon as he’s free, fingers racing to undo the buckles of your harness. “You didn’t come.” He huffs, tugging at the straps, “I wanna make you come. Please let me.”

You shove the strap-on away, throwing it half-way across the room. “How do you want me, Keishin?”

Keishin collapses, rubbery, on the bed. “Sit on my face, Princess.”

Fuck. You can do that. You move up until your knees bracket his head and hold yourself over his face. “Fuck, you’re so wet,” he whispers, kissing the dampness from your thighs, working up to your center.

He licks into you delicately, mopping up all of your juices. You’re hypersensitive already and gasp into his teasing touches. Keishin slides his tongue inside you, curling it upwards. You keen, grinding down onto his mouth before you can stop yourself. You move to pull off to apologize, but Keishin holds your hips down, face more blissful than you’ve ever seen it. You run your fingers through his hair, swiveling your hips over his mouth.

“Need you on my clit,” you gasp and Keishin hums (which, okay, wow) and sucks your clit between his lips, sliding two thick fingers into you. He licks and sucks at you, pushing you farther and farther closer to the edge, but it’s the gentle nibble that finally pushes you over it. You scream soundlessly, fingers scrambling for purchase on the bed. His hands keep you from falling off his mouth as he licks you down from your orgasm. When you mewl in discomfort, he presses one last kiss to you clit before pulling away.

You collapse next to him, thighs sore and blissed out.

“Learn your lesson?” you asked him sleepily, eyes closing.

“No wonder none of the boys are failing chemistry. You’re quite the teacher,” Keishin nods, still panting slightly. “Though, I think you may have to go over it again sometime.”

You laugh and turn to look at him. He’s smiling back at you, eyes soft and happy. The heavy feeling in your chest returns, and you feel like you can’t breathe. You lean in and kiss him, ignoring the way he tastes like you. His own flavor was much sweeter. “I think we can manage that,” you whisper against his glistening lips.

He lazily tangles his hand in yours and brings it up to kiss you knuckles. “Good.”

When you wake the next morning with muscular forearms wrapped around you, you panic for a moment before remembering who it is and relax into Keishin’s embrace.

**Author's Note:**

> hello hi i’m only on episode 19 of haikyuu so idk how in character this is 
> 
> also yes i know that newton’s law is physics but i made mc a chemstry teacher because 1) physics is the worst of all high school sciences and 2) all chemistry teachers are inherently sexy


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